The part of you that defines itself as a personality, separates itself from the outside world, and considers itself (read: you) a separate entity from the rest of nature and the cosmos. Perhaps necessary for survival in some evolutionary bygone, in modern times it leads only to (albeit often disguised) misanthropic beliefs and delusion.

For materialist world, the ego could provide the evolutive energy but also the destructive energy depends on how you deal with it. For spiritual world, there is no need for ego, there we are only one entity.

Some examples:
possitive ego gives you ambitious to be the best in your field
the negative ego make you to think only to yourself.

Something that most people who add this word to Urban Dictionary, get wrong.

Ego roughly literally translates as the conscious mind or consciousness of your own identity.

Ego is something that people too often mistaken as a bad thing. If what the majority says is true, then I guess you can say that you're conscious mind or consciousness of your own identity is considered a terrible thing to have.

A song by Beyonce, where she cleverly talks about a certain someones dick size. She repeatedly mentions how his "Ego" is: too tough, too strong, too big, too wide, and how it won't fit. I think Jay-Z is taking viagra. And in the remix, Kanye West throws in how his "hahaha ego" is so big, and he wants her to "stroke my big hahaha ego". Nice. But I do love the song. =D